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Blue-Eyed Humans Have a Single, Common Ancestor

Blue-eyed humans have a single, common ancestor

New research shows that people with blue eyes have a single, common ancestor. Scientists have tracked down a genetic mutation which took place 6,000-10,000 years ago and is the cause of the eye color of all blue-eyed humans alive on the planet today.

What is the genetic mutation"Originally, we all had brown eyes," said Professor Hans Eiberg from the Department of Cellular and Molecular Medicine. "But a genetic mutation affecting the OCA2 gene in our chromosomes resulted in the creation of a "switch," which literally "turned off" the ability to produce brown eyes." The OCA2 gene codes for the so-called P protein, which is involved in the production of melanin, the pigment that gives colour to our hair, eyes and skin. The "switch," which is located in the gene adjacent to OCA2 does not, however, turn off the gene entirely, but rather limits its action to reducing the production of melanin in the iris -- effectively "diluting" brown eyes to blue. The switch's effect on OCA2 is very specific therefore. If the OCA2 gene had been completely destroyed or turned off, human beings would be without melanin in their hair, eyes or skin colour -- a condition known as albinism.

Limited genetic variationVariation in the colour of the eyes from brown to green can all be explained by the amount of melanin in the iris, but blue-eyed individuals only have a small degree of variation in the amount of melanin in their eyes. "From this we can conclude that all blue-eyed individuals are linked to the same ancestor," says Professor Eiberg. "They have all inherited the same switch at exactly the same spot in their DNA." Brown-eyed individuals, by contrast, have considerable individual variation in the area of their DNA that controls melanin production.Professor Eiberg and his team examined mitochondrial DNA and compared the eye colour of blue-eyed individuals in countries as diverse as Jordan, Denmark and Turkey. His findings are the latest in a decade of genetic research, which began in 1996, when Professor Eiberg first implicated the OCA2 gene as being responsible for eye colour.
Nature shuffles our genesThe mutation of brown eyes to blue represents neither a positive nor a negative mutation. It is one of several mutations such as hair colour, baldness, freckles and beauty spots, which neither increases nor reduces a human's chance of survival. As Professor Eiberg says, "it simply shows that nature is constantly shuffling the human genome, creating a genetic cocktail of human chromosomes and trying out different changes as it does so."

Nature shuffles our genesThe mutation of brown eyes to blue represents neither a positive nor a negative mutation. It is one of several mutations such as hair colour, baldness, freckles and beauty spots, which neither increases nor reduces a human's chance of survival. As Professor Eiberg says, "it simply shows that nature is constantly shuffling the human genome, creating a genetic cocktail of human chromosomes and trying out different changes as it does so."

I don't follow. How it became so predominant in a population if only a single common ancestor surrounded by a multitude of brown-eyed people had this mutation? How is this possible if "the mutation of brown eyes to blue represents neither a positive nor a negative mutation"? The decrease of melanin production must have been a very positive mutation instead, whatever the reason.

From their article, they reach their conclusion by analysing "155 blue-eyed individuals from Denmark" as well as "5 and 2 blue-eyed individuals from Turkey and Jordan, respectively." ... I'm not a geneticist but it hardly seems a representative sampling.

Anyway, so much missing pieces of the puzzle. I have the feeling that the truth lies far deeper in the past than we are told probably because we have actually no clue of the full picture passed a certain point in history. And this point is not that far. Most research are also carried on by hyperspecialists nowadays and it becomes hard for them to give conclusions that aren't valid only for their own particular field of research...

I don't follow. How it became so predominant in a population if only a single common ancestor surrounded by a multitude of brown-eyed people had this mutation? How is this possible if "the mutation of brown eyes to blue represents neither a positive nor a negative mutation"? The decrease of melanin production must have been a very positive mutation instead, whatever the reason.

Think that we were still in the paleolithic in Europe, so a mutation like this cuold have easily spread in small clans (and we are talking about millennia). The HG europeans were very few, like all HG groups in any period of human history and in any corner of the world

A high percentage of the current population of the baltic region and nordic regions makes sense, because a migration across millenials with many founder effects seems plausible, especially with the retreat of glaciers at the end of the Ice Age, and the migration to northern latitudes from this northwestern region of the Black Sea.

I don't follow. How it became so predominant in a population if only a single common ancestor surrounded by a multitude of brown-eyed people had this mutation? How is this possible if "the mutation of brown eyes to blue represents neither a positive nor a negative mutation"? The decrease of melanin production must have been a very positive mutation instead, whatever the reason.

Consider that today more humans are alive than in the entire history of humans (2-3million years) combined.
Early populations (everything pre-farming era, depending on the region 5-8k years only) were bands of families with maybe 20-30 people, wandering around. Occasionally, individuals were exchanged between the bands, and once the mutation is present in an individual, it will be inherited to the next generation, regardless of whether the individual has itself blue eyes or not. To spread through the entire living population therefore maybe didnt take longer than a hundred or two hundred years.

I would agree that the mutation occured longer ago in the context of general lesser melanin = light skin of Neanderthal populations already and not so short time ago only. Neanderthals had light skin, often red hair, often freckles and mostly green or otherwise light eyes. So the mutation probably is rather 30-40k old and resides in a section of DNA that is generally responsible for the light complexion of all Europeans today.

That it is inherited regardless of the parent's own eye colour, there are plenty of examples, just look at the Becker kids. Even though their facial features are clearly negro, some of them have blue eyes, so unlike former claims, it is apparently not a completely recessive gene that is overwritten once you breed with a brown-eyed individual. The section of the DNA is still inherited and remains intact.

But this researcher claims that it happened by random chance, "simply" by "trials" of nature and "shuffling" and doesn't represent a positive or negative mutation for survival. How come that this mutation happened just by coincidence with the migration of these people to higher latitudes? Or was this mutation already presents in some "encounters" who have lived at higher latitudes already a long time before and were wandering around as well?

Then he claims it happened for the first time ever 6 to 10k ago with a single individual: "Before we had all brown eyes"... I don't know who is "we" here and how "brown" it was but, related to the first comment and if a decrease in overall melanin production gives a survival advantage, why it couldn't have happened long before during the many Ice Age cycles while hominids were roaming across Europe? There have been at least 8 glacial cycles during the last million years alone. How do we know that this mutation wasn't present in some populations living in cold climate, even before the Neandertals who themselves didn't fall out of the blue? Is an analysis of 155 people from Denmark, 5 from Turkey and 2 from Jordan enough to prove it given our knowledge?

I have sparse knowledge on genetics and human population evolution, but I assume the researcher is a believer of the out of Africa replacement theory. This explains the “we” (= Homo sapiens who transformed themselves along the way in many successive waves and isolation from the other hominids). But just wondering how the story goes if there were countless waves of back and forth migrations of many related hominids and this not only between Homo sapiens and Neanderthals but the full spectrum of hominids who lived for millions of years across Europe, Africa and Asia…